Milankovitch Cycles Part 5

This is fifth article in a series by RCA Member David Horne.

By 1920, Milankovtich completed the book he began writing during the World War: Mathematical Theory of Heat Phenomena Produced by Solar Radiation. In this work, Milankovtich set out the mathematical tools he developed by which he calculated and described the amount of solar radiation received planet wide on Mars, Earth and Venus. As mentioned in the introduction to this series of articles, his calculations for Mars and Earth proved accurate. But, because he was unaware of Venus’ dense atmosphere his calculations of Venus’ were not even close — spacecraft probes have measured surface temperatures over 850 degrees Fahrenheit and pressures equal to the pressure 3000 feet underwater. The book received more attention from the public for his Mars’ results than it did from geologists interested in Earth’s Ice Ages because in the first decades of the 20th Century, the popular press was filled with stories about Martians and canals on Mars.

Finally, though, Milankovitch’s book came to the attention of Wladimier Koeppen, the famous German meteorologist. Koeppen’s daughter was married to Alfred Wegner, the famous geologist responsible for the theory of plate tectonics. Koeppen and Wegner were working on a book on past climates when Koeppen came across Milankovitch’s work. The three scientists formed a friendship that lasted until Wegner’s death during an expedition in Greenland in 1930 and that of Keoppen’s in 1940.

Initially, Milankovitch had calculated temperatures back 160,000 years. But with the help and encouragement of Koeppen, Milankovitch calculated temperatures back 650,000 years at 10,000-year intervals, for the latitudes of 55 degrees, 60 degrees, and 65 degrees. It took him three months. The period was chosen because at the time, scientists had discovered 4 periods of glaciation dating back 600,000 years. Science has since discovered more glacial epochs. Here is a copy of the chart showing the four glacial ages then known, and the corresponding to Milankovitch’s predicted solar radiation values by latitude (Figure 1). His calculations predicted additional glacial epochs.

He continued his work and by 1930, Milankovitch had computed by hand the curves showing the amount of radiation varying over time at eight latitudes from 5 degrees to 75 degrees both north and south of the equator.

Milankovitch, Koepen and Wegner published their results in 1924, and Milankovitch continuing to work on the problem. He published his final work, a compendium of all his writings, in 1941 in occupied Europe: Canon of Insolation and the Ice Age Problem.

The three scientists working together, based upon Milankovitch’s calculations, reached the following conclusions. First, Croll and Adhemar (from part IV of this series) believed it was the amount of snow deposited each winter that was the key factor. But by the late 19th Century, several scientists speculated that it was summer temperatures that were the key. It didn’t matter how much winter snow fell if the summer was sufficiently hot to melt it. Milankovitch, Wegner and Keopen concluded that it was summer, not winter, that was the critical season in the development of a Glacial Age. Glaciers developed when sufficient snow and ice cover carried over from one year to the next. So colder summers meant more accumulation from year to year. And, they believed that since ice sheets first developed and last melted at 65 degrees North Latitude, this is the most sensitive latitude for the onset of a glacial epoch.

Focusing on lower summer temperatures in the higher latitudes above 65 degrees north also meant that axial tilt cycle (Part III of this series) operating on a roughly 41,000-year cycle, had the greatest impact on Summer temperatures in the Northern or higher latitudes. Second, recall the hula-hoop motion, apsidal precession, of the Earth’s orbit from Part III. It acts to vary the position at which the seasons occur. So instead of Winter occurring when Earth is closest to the Sun (now), in 50,000 plus years it will occur at the other end of the orbit when Earth is furthest from the Sun. This cycle completes every 112,000 years approximately. When this motion is “combined” or “beat together” with the axial precession cycle (explained in Part II), the result is an approximately 22,000-year precession cycle. This combined cycle or “beat frequency” (see Part II on ‘beat frequency’) has a larger impact or effect on summer temperatures in the lower latitudes.

This then is the core of the Milankovitch theory at least as far as the mathematical model would permit. But Milankovitch realized there were holes, questions unanswered. One of the most important questions was whether the changes in insolation were large enough to trigger the onset of glacial formation. He needed a way to test his calculations in the real world, and he hit upon an ingenious method. Modern methods for determining past climate temperatures did not exist. Milankovitch though, knew that the elevation of the permanent snow line in the high mountains is very sensitive to temperature changes and that the snow line elevation varies with latitude. He also knew that snow and ice reflect solar radiation much better than open ground, more snow means more solar energy reflected into space making it easier for additional snow to accumulate. Croll was aware of this reflectivity issue; he had theorized that such positive feedback mechanisms were involved in the onset of an ice age. But he lacked the data to formulate a theory.

By the 1930s measurements on snow and ice reflectivity had been made. Milankovitch plugged them into his calculations and it worked: he was able to relate the changes in insolation (the amount of radiation received from the Sun) to the onset of glacial formation. When the geological data showed that snow lines had descended to sea level Milankovitch knew that an ice cap, glaciers, were present. He now had the means to track the location and onset of glacial epochs. The following two graphs help visualize Milankovitch’s results.

The first graph (Figure 2) is based upon ice core samples obtained using modern techniques calculated for 65 degrees North Latitude, showing temperature over time against versus solar radiation.

The next chart is of the radiation curves predicted by Milankovitch in his 1941 book set against measurements by two modern climatologists active in the field. It shows the cyclical variations in radiation with the Glacial Epochs noted at the top by the names given the epochs by geologists. The correspondence with Milankovitch’s predicted results is obvious.

Figure 3. From: Cyclostratigraphy and its revolutionizing applications in the earth and planetary sciences; Linda A. Hinnov, Geo Science World, GSA Bulletin DOI: 10.1130/B30934.1 Published on November 2013, First Published on October 24, 2013

The predicted values from Milankovitch agreed well with the information about the Glacial Epochs scientists had at the time. By 1950, the theory had been generally accepted. But in the years following World War II, new techniques for dating past climates and temperatures were being developed. The techniques have continued to develop and information has continued to flow in, and this is where theory meets reality. The next chapter, In Reality, we will look at what scientists have found when they went looking for evidence of the cycles in our world

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